The present invention generally relates to development methods of electrostatic latent images and more particularly, to a reversal development method of electrostatic latent images by the use of high-resistivity magnetic toner.
Conventionally, there has been proposed a reversal development method for practical applications by the use of electrically conductive magnetic toner, whereby a developing device can be made simple in structure and easy in maintenance, with elimination of problems related to deterioration of developer due to the fatigue. However, in the case where the known reversal development method is applied to copying apparatuses of toner image transfer type, serious problems as follows may arise. Namely, in the case where the known reversal development method using the electrically conductive magnetic toner is applied to copying apparatuses of the above-described type, the development by the electrically conductive magnetic toner is performed through an electric charge injecting phenomenon, so that the toner which is caused to adhere to the image portion of the electrostatic latent image by the development is formed into one layer and thus, the development maintains the toner image, i.e. the developed image stable in quality but is limited, in its application, to development of low density images. Furthermore, since the toner itself is electrically conductive, the palarity of the toner is readily caused to change during transfer of the toner image, and a so-called "blow off" phenomenon takes place in which the toner once transferred onto the transfer paper sheet is separated from the transfer paper sheet, thereby resulting in a reduction in the transfer efficiency or indefinite transferred images.
Accordingly, as a matter of fact, it is generally considered necessary at present that the toner to be used for the reversal development which may be applied to the above described copying apparatus, should be electrically insulative or highly resistant. Therefore, for making it possible to effect the reversal development so as to obtain a positive image from a negative original, while employing the electrically insulative or high-resistivity toner, it has been inevitable to selectively adopt one system (1) in which the polarity of the electrostatic latent image formed on a surface of an electrostatic latent image carrier, i.e. charged polarity on the surface of the photoreceptor is changed over, or another system (2) in which toners having different charged polarities are changed over for use in a dual component developing material composed of a mixture of carrier and toner, with respect to the normal development for obtaining a positive image from a positive original.
However, since, in the above-described reversal development methods (1) and (2), the toner is caused to adhere to a portion (i.e. an image portion, more specifically a portion corresponding to a white image of the original), from which the electric charge of the latent image is erased, through electrostatic repulsive force, fogging of toner cannot be avoided. Furthermore, the method (1) has a disadvantage that the sensitivity of the photoreceptor varies in the cases of the normal development and the reversal development, while in the method (2), there is an inconvenience that the copying apparatuses are required to be exclusively used as special purposes apparatus for the reversal development because it is practically difficult to use two kinds of toner through changeover thereof.